Miniaturized gas sensor device and method

ABSTRACT

Various embodiments of a gas sensor device and method of fabricating a gas sensor device are provided. In one embodiment a gas sensor device includes a base substrate, an electrolyte layer disposed on the base substrate and a plurality of potentiometric sensor units electrically coupled to the base substrate. Each potentiometric sensor unit includes an electrolyte layer disposed on the base substrate, a sensing electrode comprising tungsten oxide (WO 3 )and platinum (Pt), a reference electrode comprising Pt, and a plurality of connectors coupled to the plurality of potentiometric sensors to connect the plurality of potentiometric sensors in series.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/212,006, filed Mar. 14, 2014 which claims priority to ApplicationSer. No. 61/801,106 entitled “Miniaturized Gas Sensor and Method” filedon Mar. 15, 2013, the entirety of which is incorporated by referenceherein.

TECHNICAL FIELD

The embodiments of the present invention relate generally to gassensors. More specifically, the disclosure relates to miniaturized gassensors that detect NO_(x) gas.

BACKGROUND OF THE INVENTION

Nitric oxide (NO) sensing is a critical capability for a variety ofapplications ranging from high temperature combustion to clinicalanalysis. In high temperature combustion applications, detection ofnitrogen oxides (NO_(x)) is critical in controlling the processes usedto reduce the NO_(x) emissions produced by the leaner combustionprocesses being developed to improve fuel efficiency. NO_(x) sensorsthat are high temperature capable may also find use in otherhigh-temperature applications. Another area where NO_(x) sensing isrequired is in the medical industry, specifically in breath analysis.These do not typically involve applications where the sensor operates ina high temperature ambient environment, but it is one where thedetection of nitric oxide (NO) itself has high importance.

There are a variety of ways to detect NO, with solid-stateelectrochemical sensors being one such technique. Such sensors also havethe added benefit of being easier to miniaturize compared to othertechniques. A variety of solid-state electrochemical sensors for NO havebeen demonstrated previously. These techniques vary and a continuingchallenge is to design sensitive systems with limited size, weight andpower consumption so as to allow for portable sensor systems. Suchadvancements would have notable impact on the healthcare industry inenabling homecare monitoring units.

NO sensors capable of detecting NO at concentrations as low as 7 ppbhave been demonstrated using an array of sensor units in series toincrease the resulting sensor signal for a given NO concentration.However, these sensors were made using hand assembly techniques and alsowere assembled into arrays by hand. This manual fabrication limits theminimum size to which the sensors can be reduced.

Miniaturized sensors based on microelectromechanical systems (MEMS)fabrication technology have been demonstrated for aerospaceapplications. Sensors made by MEMS fabrication are very small devicesthat can be made up of components and features between 1 to 100micrometers in size (0.001 to 0.1 mm). Fabrication is a challenge atthese size scales for several reasons. Large surface area to volumeratio of MEMS, and the resulting surface effects which dominate overvolume effects can improve sensor performance. However, the overallsurface area of a MEMS sensor unit may be notably smaller thancorresponding macro sensor devices. This may decrease the overall numberof chemical reactions involved, resulting in a decreased signal. Thus,improved sensor design is mandatory to enable miniaturization of sensorsystems. Such optimization may be different on the macro level then formicro sensors, and simple application of design principle that aresuccessful for macro sensor can lead to significantly degradedperformance for micro sensors.

A reduction in size of the sensors using MEMS techniques would not onlydecrease the size for better implementation in a handheld homemonitoring unit, but the reduced size would also decrease the powerrequired to bring the sensors up to operating temperature. In addition,the utilization of MEMS fabrication techniques introduces batchfabrication that allows for multiple sensors to be made at one time,thus reducing costs.

SUMMARY

Various embodiments of a microfabricated gas sensor device and method offabricating a miniaturized gas sensor device are provided. In oneembodiment a microfabricated gas sensor device includes a basesubstrate, an electrolyte layer disposed on the base substrate and aplurality of potentiometric sensor units electrically coupled togetheron the base substrate. Each potentiometric sensor unit includes anelectrolyte layer disposed on the base substrate, a sensing electrodecomprising tungsten oxide (WO₃), a reference electrode comprisingplatinum (Pt), and a plurality of connectors coupled to the plurality ofpotentiometric sensors to connect the plurality of potentiometricsensors in series. The structure of each of these potentiometric sensorunits is designed to greatly improve sensor response.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments of the present invention can be understood withreference to the following drawings. The components in the drawings arenot necessarily to scale. Also, in the drawings, like reference numeralsdesignate corresponding parts throughout the several views.

FIG. 1 illustrates a miniature sensor device, which is a firstgeneration design using shadow masks, according to an embodiment of thepresent invention;

FIG. 2 illustrates a miniature sensor device, which is a secondgeneration design using shadow mask, according to an embodiment of thepresent invention;

FIGS. 3a, 3b, 4a, 4b, 5a, 5b, 6a, and 6b illustrate a sensor device atvarious steps of a fabrication process, according to an embodiment ofthe present invention;

FIG. 7 is a flow chart of the method for making the sensor device,according to an embodiment of the present invention;

FIG. 8 illustrates a photographic image taken under a microscope of oneof the sensor units in the first generation, shadow mask design of FIG.1, according to an embodiment of the present invention;

FIG. 9 illustrates aphotographic image taken under a microscope of asensor unit of the second generation, shadow mask design of FIG. 2,according to an embodiment of the present invention;

FIG. 10 illustrates a photographic image taken under a microscope of analternative miniature sensor unit of a second generation, shadow maskdesign, according to an embodiment of the present invention;

FIG. 11 illustrates the spectrum from XPS analysis of sputter depositedWO₃ film, in accordance with an embodiment of the present invention;

FIG. 12 illustrates a miniature sensors of the second generation shadowmask design, and a photoresist version of the miniature sensor comparedto a hand fabricated sensor, according to an embodiment of the presentinvention;

FIG. 13 illustrates the test results of a miniature five sensor unitdevice of the first generation, shadow mask sensor designat 50 ppmexposure to NO, according to an embodiment of the present invention;

FIG. 14 illustrates the test results of a miniature five sensor unitdevice of a second generation, shadow mask sensor design at 33 ppmexposure to NO, according to an embodiment of the present invention;

FIG. 15 illustrates test results of a miniature ten sensor unit deviceof a second generation, shadow mask design in a plot of sensor voltageresponse to 20 ppm, 50 ppm, and 100 ppm NO exposure, according to anembodiment of the present invention;

FIG. 16 illustrates the test results of a miniature fifteen sensor unitdevice of a second generation, photolithographic design in a plot ofsensor response to 0.5 ppm, 2 ppm, 5 ppm, and 10 ppm NO exposure,according to an embodiment of the present invention;

FIG. 17 illustrates a photographic image showing a six sensor unitdevice of a second generation, photolithographic design,according to anembodiment of the present invention;

FIG. 18 illustrates a photographic image showing a six sensor unitdevice of an alternative second generation, photolithographic design,according to an embodiment of the present invention;

FIGS. 19a and 19b are SEM images of sensor showing cracks of the YSZfollowing the photolithographic process, according to an embodiment ofthe present invention;

FIG. 20 illustrates a plot of test results of a sensor device beforeextended heat exposure, according to an embodiment of the presentinvention;

FIG. 21 illustrates a plot of test results after 48 hours of continuousheat exposure, according to an embodiment of the present invention;

FIGS. 22 and 23 before and after, respectively, extended heat exposure,according to an embodiment of the present invention; and

FIGS. 24a-d illustrate SEM images of sensor region containing WO3 afterheat exposure, according to an embodiment of the present invention.

DETAILED DESCRIPTION

The various embodiments of miniature NO sensors disclosed herein is anelectrochemical sensor whose structure includes sensor units of solidelectrolyte, a reference electrode and a working electrode. Anelectromotive force (EMF) is induced between the working and referenceelectrodes when NO impinges on the sensor due to the dissimilar chemicalactivity at each electrode. In one embodiment, the reference electrodeis platinum (Pt), while the sensing or working electrode is tungstenoxide (WO₃). The solid electrolyte is yttria-stabilized zirconia (YSZ).These sensor material choices are based on larger hand-made sensors thatare described further in the examples below.

FIG. 1 shows a miniature sensor device 10 having individual sensor units50 and fabricated on aluminum oxide substrates 12. Each sensor on thesubstrate comprises a YSZ island 50 upon which the rest of the sensor isbuilt. Pt reference electrode 52 and WO₃ sensing or working electrode 54lie on top of the YSZ island 51. A Pt contact 55 of the sensingelectrode is located below the WO₃ to make contact to the WO₃ of thistwo-part sensing or working electrode. As each sensor 50 is anelectrochemical cell, the sensors can be connected in series to generatea larger signal response. Thus, an array of sensors can be used toimprove the signal response to NOx, including NO. The sensors areelectrically connected together as in the cells of a battery such thatthe induced EMF's are additive, thereby increasing the response for agiven NOx concentration over a single sensor. Sensor devices of 5, 10,15 and 20 sensors were connected electrically and tested. Each sensorarray is interconnected electrically via Pt leads. This fabricationapproach represents a simple reduction in size of the larger in thesensors and it has been found herein, as demonstrated in the examplesbelow, that simple reduction in size does not result in the desiredsensitivity needed to improve sensor performance.

With reference to FIG. 1, the gas sensor device 10 includes a pluralityof, i.e. or at least two, sensor units 50 in a dice unit row 12. Each ofthe sensor units 50 includes a reference electrode 52 and a sensingelectrode 54. In one embodiment, the sensing electrodes 54 comprise WO₃,and the reference electrodes 52 comprise platinum (Pt).

In one embodiment, the gas sensor device 10 includes 3 rows 14 of sensorunits 14, but a variety of sensor units 50 is possible. Each of thesensor units 50 is electrically coupled to at least one adjacent sensorunit 50. For example, the sensor units 50 are electrically connectedtogether in series. The combined potential difference of the pluralityof sensor units 50 is approximately a sum of the potential differencesof each of the individual sensor units 50 electrically connected to oneanother.

Experimental tests that have been conducted herein show that thesensitivity of the system 10 is based on the number of sensor units 50.Each of the sensor units generates a potential difference in theresponse to a gas, for example, NO_(x) gas. Generally, a systemincluding more sensor units 50 has been found to be relatively moresensitive to NO, and a system including less sensor units has been foundto be relatively less sensitive to NO. However, there is a point atwhich additional sensor units 50 will not improve the sensitivity, andit has been found that sensor devices that have 15-20 sensor units haveincreased sensitivity. This is due to lack of previous recognition ofthe various elements of sensor design including, for example, theinternal resistance of each sensor element. As noted above, acorresponding reduction of the size of the sensor having the samematerials of construction do not result in improved sensitivity. Theimpact of the internal resistance of the individual sensor units, andthe design features that contributed to higher sensitivity is describedherein and was discovered during the course of the fabrication ofsensors as discussed in the examples below. Overcoming internalresistance is core to even higher levels of sensitivity.

It has been found herein, in accordance with various embodiments of thepresent invention, that reducing the exposed surface area of Ptreference electrode on the YSZ electrolyte and increasing the surfacearea of WO₃ electrode covering the YSZ electrolyte improves thesensitivity of the sensor as can be seen in comparing the results of thefirst generation, shadow mask design of FIG. 1 and second generation,shadow mask design of FIG. 2 and shown in the results of the examples.FIG. 2 shows a miniature sensor device having individual sensor units 60and fabricated on a substrate, for example, aluminum oxide substrates.Each sensor on the substrate comprises a YSZ island 50 upon which therest of the sensor is built. Pt reference electrode 62 and WO₃ sensingor working electrode 64 lie on top of the YSZ island 61. A Pt contact 65of the sensing electrode is located below the WO₃ to make contact to theWO₃ of this two-part sensing or working electrode. As each sensor unit60 is an electrochemical cell, the sensors can be connected in series asa sensor device to generate a larger signal response. Thus, an array ofsensors can be used to improve the signal response to NO.

The ratio of the exposed WO₃ to the exposed platinum Pt is maximized toincrease the sensitivity and obtain a low end sensor reading.Furthermore, in another embodiment, the ratio of the exposed WO₃ to theexposed platinum Pt is maximized while also decreasing the size, forexample the surface area, of the Pt contact of the sensing electrodethat is contact with WO₃. Decreasing the size of the contact 65underneath the WO₃ so that it is minimized to the extent of fabrication(i.e. within the resolution of the fabrication approach, such as 2microns) is found to increases the sensitivity of the microfabricatedsensors units 60. This electrode structure is not a simple one componentelectrode, but rather composed of both an oxide and metallic electrodecombination that together are designed for improved response. Inaccordance with an embodiment of the present invention, the electrolytelayer of the microfabriated potentiometric gas sensor device has athickness of the electrolyte layer that is maximized a sufficient amountto minimize the internal resistance of the potentiometric sensor unit,and such that the internal resistance of each of the plurality of sensorunits is minimized so as to minimize the overall resistance of thesensor device to increase the sensitivity of the sensor device.

For example, in one embodiment the surface area of the WO₃ electrode onthe electrolyte is greater than the surface area of the Pt electrode. Inanother embodiment the WO₃ covers all of the available surface on theYSZ unused by the Pt electrode within the resolution of the fabricationapproach (approximately 2 microns depending on the equipment used). Inanother embodiment the surface area of the WO₃ electrode is at least twotimes greater than the surface area of the Pt electrode, and in anotherembodiment, the WO₃ electrode is at least 5 times greater than thesurface area of the Pt electrode, and in yet another embodiment the WO₃electrode is at least 10 times greater than the surface area of the Ptelectrode.

The increased surface area of the WO₃ boundaries does increase thetriple point boundary of the WO₃ electrode, the YSZ electrolyte and thegas, for example, NO gas compared to those of the Pt electrode. Thedecreased surface area of the Pt electrode decreases the triple pointboundary of the Pt, the YSZ electrolyte and the gas. The limitation onthe amount of YSZ surface area that is not also a triple point boundaryis believed to decrease the sensitivity of the sensor. As a result ithas been found that the sensitivity of the sensor device can beincreased.

In addition FIG. 9 shows that the surface of the WO₃ electrode 310 ofsensor unit 300 has at least one lateral projection 320. The at leastone lateral projection has at least two edge interfaces 330, 340 alongthe surface of the electrolyte, and in another embodiment at least threeedge interfaces, thereby creating additional triple point boundarypoints between the WO₃ electrode and YSZ electrolyte. In anotherembodiment, FIG. 10 shows that the surface of the WO₃ electrode hasthree lateral projections 420, 430 and 440. Each lateral projection hasat least three edge interfaces 510, 520 and 530 along the surface of theelectrolyte which forms a triple point boundary between the WO₃, the YSZelectrolyte and the gas. As shown the lateral projections form cornersformed by the edge interfaces 520 and 530. These lateral projectionsincreased the torturous nature of the pattern and increase the number oftriple points. For example, these lateral surface contours increase thelineal length of edge interface between the WO₃, the electrolyte, andthe surrounding gas as illustrated in FIGS. 9 and 10.

Further, the size of the YSZ patterns in FIG. 2 compared to FIG. 1decreases the distance, and thus corresponding resistance, between theelectrodes. Other factors decreasing this resistance include thethickness of the electroylyte, yttria-stabilized-zirconia, and thicknessof the various metal layers that are micro-deposited on the surface.Such features are not apparent in macro sensor since, for example, a 5micron change in the thickness of the zirconia in a macro sensor hasmuch less effect on the internal resistance of the sensor unit where itwould nearly eliminate the zirconia layer for a microfabricated sensor.

It should be noted that this is a potentiometric sensor (voltagedifference), rather than an amperiometric sensor (current flow). In asensor that measures current flow, the effect of resistance is known andtoo large a resistance can readily be seen to limit the measurement.Such amperometric sensors are not linked in series like batteries (asare the potentiometric sensors in our work) and the effect of increaseresistance is directly noticeable in the measurement. It is discoveredthat an aspect of the potentiometric sensor device that includes sensorunits linked in series, is that high resistance of each sensor unit wasfound to limit the lower detection limit of the sensor overall. Thus,while each sensor unit might have a resistance that did not notablyaffect its operation; the combined resistance of each of thepotentiometric sensor unit in series can change the lower limitdetection capabilities of the overall sensor. This may not be obvious athigher concentrations, but was found to have significant effect onsensitivity for lower concentration measurements. It was found thatdecreasing this overall resistance is a feature of increasing thesensor's lower detection limit.

The microfabricated potentiometric gas sensor device senses gas at abroad range of temperatures, including but not limited to, hightemperatures that range from about 500° C. to about 700° C., in anotherembodiment, from about 550° C. to about 650° C.

Method of Fabrication

MEMS fabrication has been successfully implemented in the examplesherein, where sensors are batch fabricated on a single wafer, with eachwafer containing multiple sensors units. These examples show thatapplying the concepts above are not only achievable but improve thecapability of the sensor. These examples are meant to show differentaspects of the design optimization from large to smaller sensors and sowhile one example may show improved response time but decreasedresponse, it is the combination of the various design features that isunderstood to provide an improved sensor system, or may be used asneeded to emphasize certain aspects of the sensor response. The sensorsare fabricated using masks and thin film deposition techniques. Eachlayer of the structure is deposited via sputtering from a targetcontaining the desired material or a component of the desired material,with the masks serving to define the shape of the resulting depositedfilm. The fabrication of these sensors was carried out using thin metal,shadow masks or photoresist masks. The sensors were fabricated ontwo-inch alumina wafers.

The general process flow for the sensors is shown in FIG. 7. The firststep in the fabrication process is to clean the alumina wafer using acombination of solvents, generally acetone followed by isopropanol. TheYSZ islands are sputter deposited using the first mask, formingindividual rectangular islands of YSZ. A thermal anneal is then carriedout at 1000° C. for two hours in air ambient to densify the YSZ. The Ptelectrodes are then deposited using the next mask. A second platinumlayer is sputter deposited to form the interconnects, electricallyconnecting the sensors in the array. It should be noted that in somedesigns of the sensor device the Pt interconnects are deposited at thesame time as the first Pt electrode deposition. Finally, the WO₃ isreactively sputter deposited from a tungsten target in an argon/oxygenatmosphere using the third mask. The sensors are then diced intoindividual arrays following the final film deposition.

The three films that are deposited are the YSZ, Pt, and WO₃ films. Boththe YSZ and Pt films are deposited by a sputter deposition process. TheWO3 also is deposited by a sputter process. However, the sputter processis a reactive sputter process using a tungsten (W) target whereby a Wtarget is sputtered to produce W atoms that are then reacted with anoxygen gas flow in the chamber prior to impingement on the substratewhere they are deposited as WO₃. The deposition is done at roomtemperature using a cooled substrate to keep the substrate cool. XPSanalysis on the films confirmed the proper stoichiometry of the filmsafter the sputter deposition processes, as shown in FIG. 10. No sensorsfabricated using thin film microfabrication techniques that aresignificantly smaller than such sensors are herein demonstrated. Changesmade from the initial design to the size of both the reference andworking electrode as well as the contacting Pt electrode under theworking electrode resulted in improved sensitivity of the sensor. Thesesensors have demonstrated sensitivity below the ppm level.

FIGS. 3a, 3b, 4a, 4b, 5a, 5b, 6a, and 6b illustrate a sensor array atvarious steps of a fabrication process, according to an embodiment ofthe present invention. Cross-sectional and top views are shown left andright, respectively: (a) Alumina substrate (FIGS. 3a and 3b ), (b)deposition of YSZ islands (FIGS. 4a and 4b ), (c) following depositionof Pt electrodes (FIGS. 5a and 5b ), (d) sensor after deposition of WO₃(FIGS. 6a and 6b ), according to an embodiment of the present invention;

FIGS. 3A and 3B show the base layer of alumina used in the substrate 100in an embodiment of the present invention. FIGS. 4A and 4B show a sideview and top view, respectively, of the alumina substrate 100 with anelectrolyte layer 102 deposited on the substrate. FIGS. 5A and 5B show aside view and top view, respectively, of the substrate 100 with theelectrodes 104 deposited on top of the electrolyte layer 102. FIGS. 6Aand 6B show the side view and top view of the tungsten oxide WO₃electrode 106 which is the working electrode deposited on top of the Ptcontact 104.

FIG. 7 illustrates the process step for making the gas sensor. In oneembodiment the box 205 shows the first step of cleaning the aluminawafer substrate. Solvents or combinations of solvents, for examplealcohols acetone and isopropanol, or an application of solvent in aseries of acetone followed by isopropanol can be used to clean thesubstrate. In box 210 the mask is placed on the substrate, a thin metalmask for the shadow mask design and a photoresist mask in thephotoresist design n box 210, for the photoresist design the photoresist mask is applied to the surface of the substrate. Once thephotoresist mask is deposited, the substrate is soft baked to removesome of the solvents in the liquid photoresist. Then the photoresist isplaced under a UV light source and the UV light is selectively passedthrough a glass mask with defined openings through which light may passto the photoresist. Depending on whether the photo resist is positive ornegative, the resulting regions exposed to light will either become moresoluble or less soluble respectively after exposure to the UV light. Thesubstrate is then placed into a developing solution that removes themore soluble regions of the photo resist. The substrate is then placedunder heat for a hard bake of the photo resist mask. In the next step ofbox 215 the process includes depositing a layer of electrolyte using asputter deposition of the desired film. The photo resist is then removedby a solvent, usually acetone, and the sputtered film on top of thephoto resist is also removed leaving behind the thin film that wasdefined by the openings in the photo resist mask. The photo resist maskhas precisely defined features down to below 100 micrometers which is amuch smaller resolution than the shadow mask used in the embodimentsdescribed above. Next in box 220 the substrates were annealed in thepresence of oxygen in order to remove or clean the residual photo resistfrom the surface. It was found that standard techniques to remove thephoto resist perform poorly and the miniaturization process did notprovide good results compared to the shadow mask method of making thesensor when using the standard software-based photoresist removaltechniques. The oxygen annealing temperature to remove the residualphoto resist on the surface or in the pores of the films of the sensorcan be carried out at a temperature that is at least 350° C., andanother embodiment from about 350° C. to about 450° C., and in anotherembodiment from about 390° C. to 425° C. It has been found that residualphotoresist can notably affect sensor response and the oxygen annealingstep, in accordance to an embodiment of the present invention was foundto improve sensitivity.

Still referring to FIG. 7, the process for making the gas sensor furtherincludes applying another mask over the electrolyte layers and next inbox 230 the process further includes depositing the reference electrodematerial and contact material for the sense electrode over the mask butthrough sputtering. The mask is removed and for the photoresist-basedsensor the substrate is again annealed to remove residual mask materialin box 235. Next in box 240 another mask is again applied over thesubstrate and over the electrodes to deposit the connectors in box 250.In another embodiment the reference electrode, contact for the senseelectrode, and the connectors can be applied in a single step afterapplication of the mask in box 210. Next in box 260 the mask is removedand for the photoresist-based sensor the substrate then undergoes theannealing process to remove the residual photoresist. Next in box 265the mask is applied again and then in box 270 the sensing electrode isdeposited by reactive sputtering. Next the mask is removed and for thephotoresist-based sensor the substrate undergoes another annealingprocess as shown in box 270 to remove all residual photoresist. Finally,in box 280 the substrate can undergo dicing to separate the individualsensor arrays 10 on the substrate. The oxygen annealing temperatures ofboxes 235, 260, and 275 are carried out and are the same as thetemperature ranges described above with respect to box 220.

The results of the testing on these various generations of designsindicate that the changes that were made between each generation wereindeed beneficial to the overall performance of the sensor. Reducing theexposed Pt on the YSZ and increasing the WO3 covering the YSZ improvedthe sensitivity of the sensor as can be seen in comparing the results ofthe first and second generation shadow mask designs. The resultingdesign changes were applied to the photomask-based design, which isbasically the second generation shadow mask design reduced in size by afactor of 0.35. The test results of the photoresist design indicate thatthe sensor array should be capable of sensing down to at least 500 ppblevel, and in another embodiment down to about 300 ppb level.

In another embodiment, the several embodiments of the NO sensor devicedescribed above can be used in an apparatus for measuring the level ofNO. The apparatus includes the sensor device and an inlet for receivinga gas sample. The gas sample, for example, NO gas, is in fluidcommunication with the sensor. The potential difference is indicative ofa level of NO within the original sample. In one embodiment, the gassample is a breath sample from the subject. In another embodiment, thegas sample that enters the apparatus may be treated by humidification ordehumidification to improve the sensitivity. The potential difference ofthe sensor array 10 is a summation of the individual potentialdifferences across the individual sensor units in response to presenceof the NO in the gas sample.

EXAMPLES Examples—Shadow Mask Fabrication

The shadow mask version of the sensor arrays utilized a metal shadowmask during each of the deposition processes to define the depositedfilms into the desired features. The metal masks were placed onto thesubstrate and clamped at the edges of the substrate. The shadow masksare easy to use and are aligned from one mask layer to the next.However, the defined features are rather large in size, there can bedistortion in the shadow mask resulting in edges of the defined shapesthat are not sharp, and after multiple uses the resulting film build upon the shadow masks may cause warpage of the shadow mask and/ormicromasking during the deposition process as particles fall off of theshadow mask and land on the openings defined by the mask.

For comparison, our initial sensor design is shown in FIG. 1 withcorresponding dimensions, along with the second generation, shadow massthat is illustrated in FIG. 2. FIG. 1 shows that the electrolyte island51 dimension A is 1.55 mm by 2.88 mm; the Pt reference electrode 52 ofdimension B is 1 mm×0.21 mm; the Pt interconnect 53 of dimension C hasan outer length of 1.56mm and an inner length of 1.15 mm; the sensingelectrode 54 of WO₃ has a dimension D of 1.1 mm×0.31 mm; and the Ptcontact 55 of the sensing electrode E is approximately the same surfacearea as the reference electrode 52. FIG. 2 shows that the electrolyteisland 61 dimension N is 1.328 mm by 1.55 mm; the Pt reference electrode62 of dimension I is 0.4 mm×0.21 mm; the Pt interconnect 63 of dimensionG has dimensions of 0.33 mm×1.4 mm; the WO₃of sensing electrode 54 hasan “L” shape that can be calculated by dimensions L (0.54 mm), M (1.4mm), O (1.22), and K (0.88); and the Pt contact 65 f the sensingelectrode has dimensions H, 1.5 mm×0.1 mm.

FIG. 12 illustrates a comparison of sensors to hand fabricated sensor onthe far right, with a quarter shown for size reference. The shadow maskversion of the sensor is to the left of the quarter and is 10 mm by 12mm. The photoresist version of the sensor is to the far left and is 3 mmby 4 mm in size.

In comparing the first and second generation shadow mask designs, thesize of the WO₃ covering the YSZ is increased by a factor of 4.0 from0.341 mm² to 1.35 mm² for the WO₃ on the YSZ in moving from designgeneration one to two. Similarly the Pt reference electrode wasdecreased in size by a factor of 2.5 from 0.21 mm² to 0.084 mm² inmoving from design generation one to two. The size of the YSZ island foreach individual sensor is 1.55 by 2.28 mm² for generation one and 1.328by 1.55 mm² for generation two designs. These designs are shown in FIGS.1 and 2.

Examples—Photoresist Process

The second variation of the sensors that was fabricated was aphotoresist-based version of the sensors. In this version of the sensorarrays, the deposited films are defined by photoresist layers depositedon the substrate. In the photoresist process a liquid photoresist filmis spun onto the surface of the substrate. Once the photoresist is softbaked to remove some of the solvents in the liquid photoresist, thesubstrate is placed under a UV light source that is defined by a glassmask. A thin metal film on the glass mask defines openings through whichlight may pass to the substrate. Depending on whether the photoresist ispositive or negative, the resulting regions exposed to light will eitherbecome more soluble or less soluble, respectively, after exposure to theUV light. The substrate is then placed into a developer solution thatremoves the more soluble regions of the photoresist. After a hard bakethe photoresist mask is ready to be used as the mask for sputterdeposition of the desired film. Once the sputter deposition process isfinished the sputtered film is defined by a lift-off process whereby thephotoresist is removed by a solvent, usually acetone, and the sputteredfilm on top of the photoresist is also removed leaving behind the thinfilm that was defined by the openings in the photoresist mask.

The advantage of the photoresist mask versus the shadow mask is that thephotoresist mask can define features to a much smaller resolution (e.g.,down to about 2 micrometers). The photoresist version of the sensor wasdecreased to 0.35 times the size of the second generation shadow maskversion, the sensors are otherwise identical in layout design. Thedownside to the photoresist mask is the possible contamination of theunderlying materials with photoresist if they are porous. If thephotoresist is not completely removed following the thin film depositionthe resulting remnants may react at higher temperatures and form abarrier to gas reaction at the surfaces of the sensor. In fact, sensorsthat were initially fabricated using standard photoresist developmentand removal techniques performed poorly compared to the shadow maskversions. This link between photoresist contamination and degradedsensor performance was confirmed when shadow mask versions of thesensors that were covered with photoresist prior to dicing intoindividual arrays by the dicing saw were found to perform poorlycompared to similar shadow mask sensors not subjected photoresistcoating that were partially diced (pre-scribed) prior to fabrication.Several methodologies were attempted in removing any residualphotoresist from the sensor surfaces. Continued solution in acetone andapplication of ultrasonic in acetone solution were both tried withlittle change in results. A typical solution to such a problem is to usean oxygen plasma clean to remove such residual photoresist. However, dueto the nature of the films that were depositing, there was a smallpercentage of Na in the resulting films that precluded the employment ofthis oxygen plasma system as this was designated a MOS piece ofequipment that should be free of exposure to salt containing films.Surprisingly, it was discovered that when the sample substrates wereexposed to an oxygen annealing in a tube furnace after each photoresiststep, residual photoresist was removed. This higher temperature processof about 400° C. removed the residual resist on the surface or in thepores of the films on the sensors. Sensors fabricated using this methodproduced the same or better results compared to equivalent shadow masksensor arrays validating the employment of oxygen anneal to removeresidual photoresist.

FIG. 12 is an optical image comparing arrays of sensors from handfabricated down to photoresist based sensors. Each successive version ofthe sensor array is smaller in size, moving from the original hand builtsensor array to the shadow mask version to the final photoresist-basedversion. As can be seen in FIG. 12, the photoresist sensor is over amagnitude smaller in each dimension compared to the hand fabricatedversion. The reduction in size allows for smaller heater stages to beused to heat the sensor and thus reduced power requirements.

Sensor Performance—Shadow Mask

Sensors were tested at temperatures ranging from 500 to 600 degreesCelsius to determine the efficacy of the sensors. Generally sensors werefound to work best when operated between 550 and 600 degrees Celsius.The sensors were tested by bringing the sensors to temperature andawaiting the sensor's stabilization. Once the temperature was stable,various NO gas concentrations were introduced to the sensor. Air wasgenerally used as the baseline gas for these experiments. In general,the electrical connections to the sensors were made via either probetips or wires attached to the contact pads at either end of the sensorarray.

Test results of the initial first generation design are shown in FIG.13. The tested sensor was a five sensor array. As can be seen from theresults, the sensor response at 50 ppm was less than 10 mV for thisconcentration of NO. A photographic image of one of the sensors in thisarray is shown in FIG. 8. For comparison in FIG. 14, the results oftesting on a second generation shadow mask design are shown for a fivesensor array. This second generation five sensor array shows a responseof nearly 10 mV for a 33 ppm concentration of NO. It can be deduced fromthese measurements that the sensor response increased quitesignificantly (showing an equivalent response for a concentrationreduction of 1.5 times) due to the changes made from the first to thesecond generation shadow mask design. Also shown in FIGS. 9&10 arephotographic images of two sensors, showing the two different designs ofthe sensor in the second generation design. FIG. 15 shows the results oftesting with a 10 sensor array from this same wafer. The results areshown for the sensor tested at 550 degrees Celsius.

In addition it was found that sensor response of each individual sensorin the array could be maximized by applying and/or modifying severalparameters. For the working electrode minimizing the Pt exposed on theYSZ and maximizing WO₃ film exposure were found to increase the sensorresponse for a given NO concentration. Pt exposure on YSZ was minimizedat both the reference and working electrodes. This change was done tominimize the triple point boundaries between the gas, Pt, and YSZ andthus the reactions at the exposed Pt surfaces on YSZ, thereby decreasingcompeting reactions that would decrease the induced potential across thesensor. Similarly, it was found that maximizing the WO₃ film on top ofthe YSZ was found to increase the induced potential across the sensor.In this case, this was due to an increase in the number of triple-pointboundaries between the gas, WO₃, and YSZ.

Sensor Performance—Photoresist

The results of the photoresist-based version are shown in FIG. 16. Ascan be seen in the test results, the photoresist-based version of thesensor array was capable of reaching below 500 ppb. These results arefrom a sensor array of 15 sensors connected in series on a chip. Thesesensors were tested using a catalytic filter to improve signal responseand remove possible interfering gases.

The results of the testing on these various generations of designsindicate that the changes that were made between each generation wereindeed beneficial to the overall performance of the sensor. Reducing theexposed Pt on the YSZ and increasing the WO3 covering the YSZ improvedthe sensitivity of the sensor as can be seen in comparing the results ofthe first and second generation shadow mask designs. The resultingdesign changes were applied to the photomask-based design, which isbasically the second generation shadow mask design that was a factor of0.35 in size compared to the shadow mask design. The test results of thephotoresist design indicate that the sensor device is capable of atleast 500 ppb level sensitivity and lower.

It was found during testing of the sensor arrays is the increasedimpedance of the connected connector array. In general, the 15 sensorarray was found to be in the 60 MOhm range at operating temperature. Thehigh impedance made the sensor array very sensitive to electrical Noisein the surrounding environment. A thicker YSZ film can decrease theimpedance of the sensor due to the thicker film increasing the areathrough which ions could move from one end to the other of the sensor,however, it is found that the residual stress in the YSZ film increasesand at higher thicknesses this stress can cause cracking in the film,especially after a thermal exertion to the operating temperature of thesensor. These cracks often run through the film, as seen in FIGS.24a-24d , and could possibly result in lower conductivity of the film.

Another possible issue is the longevity of the sensors. Although thesensors were capable of repeated performance during testing it was foundthat over a longer period of time (several days of continuous testing)that the sensor performance would gradually decrease. From optical andSEM examination it appears that the films may be reacting at temperatureand migrating from their original deposited locations.

Although the invention has been described with reference to severalspecific embodiments, this description is not meant to be construed in alimited sense. Various modifications of the disclosed embodiments, aswell as alternative embodiments of the invention will become apparent topersons skilled in the art upon the reference to the description of theinvention. It is, therefore, contemplated that the appended claims willcover such modifications that fall within the scope of the invention.

Having described the invention, we claim:
 1. A microfabricatedpotentiometric gas sensor device comprising: a base substrate; anelectrolyte layer disposed on the base substrate; a plurality ofpotentiometric sensor units connected in series and coupled to the basesubstrate, each potentiometric sensor unit comprising: an electrolytelayer disposed on the base substrate; a two-part sensing electrodecomprising a layer of tungsten oxide (WO₃) disposed on a platinum (Pt)contact; a reference electrode comprising platinum (Pt); and wherein theratio of the surface area of the tungsten oxide (WO₃) of the sensingelectrode disposed on the electrolyte to the surface area of the Pt ofthe reference electrode disposed on the electrolyte is at least 2 to 1.2. The microfabricated potentiometric gas sensor device of claim 1,wherein the sensor device is a MEMS sensor device.
 3. Themicrofabricated potentiometric gas sensor device of claim 1, wherein thesensor device is capable of determining the gas level at a sensitivityof at least 1 ppm.
 4. The microfabricated potentiometric gas sensordevice of claim 1, wherein the sensor device is capable of determiningthe gas level at a sensitivity of at least 500 ppb.
 5. Themicrofabricated potentiometric gas sensor device of claim 1, wherein thesensor device is capable of determining the gas level at a sensitivityof at least 300 ppb.
 6. The microfabricated potentiometric gas sensordevice of claim 1, wherein the sensor device is capable of determiningthe gas level of NO_(x) at a sensitivity of at least 300 ppb.
 7. Themicrofabricated potentiometric gas sensor device of claim 1, wherein theelectrolyte comprises yttria-stabilized zirconia (YSZ).
 8. Themicrofabricated potentiometric gas sensor device of claim 1, wherein thesubstrate comprises a material that is an insulator.
 9. Themicrofabriated potentiometric gas sensor device of claim 1, wherein athickness of the electrolyte layer is maximized a sufficient amount tominimize the internal resistance of the corresponding potentiometricsensor unit, and such that the internal resistance of each of theplurality of potentiometric sensor units is minimized so as to minimizethe overall resistance of the microfabricated potentiometric gas sensordevice and increase sensitivity of the microfabricated potentiometricgas sensor device.
 10. The microfabricated gas sensor device of claim 1,wherein the exposed surface of the electrolyte layer is minimized asufficient amount to result in increased sensitivity of themicrofabricated potentiometric gas sensor device.
 11. Themicrofabricated gas sensor device of claim 1, wherein the electrolyte isYSZ and the surface area of the WO₃ sensing electrode on the electrolyteis fabricated using microfabrication techniques so as to maximize theratio to that of the WO₃ to the Pt reference electrode on theelectrolyte while minimizing the exposed layer of electrolyte layer. 12.The microfabricated gas sensor device of claim 1, wherein the ratio ofthe surface area of the WO₃ sensing electrode to the surface area of thePt contact is sufficiently high to increase the sensitivity of themicrofabricated potentiometric gas sensor device.
 13. Themicrofabricated gas sensor device of claim 10, wherein the ratio of thesurface area of the WO₃ sensing electrode to the surface area of the Ptcontact is sufficiently high to increase the sensitivity of themicrofabricated potentiometric gas sensor device.
 14. Themicrofabricated sensor device of claim 13, wherein a thickness of theelectrolyte layer of each sensor unit is maximized a sufficient amountto minimize the internal resistance of the corresponding potentiometricsensor unit, and such that the internal resistance of each of theplurality of potentiometric sensor units is minimized so as to minimizethe overall resistance of the microfabricated potentiometric gas sensordevice and increase sensitivity of the microfabricated potentiometricgas sensor device.
 15. The gas sensor device of claim 1, wherein thesurface area of the WO₃ electrode on the electrolyte is at least tentimes greater than the surface area of the Pt electrode on theelectrolyte.
 16. The microfabricated potentiometric gas sensor device ofclaim 1, wherein the surface area of the WO₃ electrode on theelectrolyte is at least five times greater than the surface area of thePt electrode on the electrolyte.
 17. The microfabricated potentiometricgas sensor device of claim 1, wherein the layer of the WO₃ of at leastone of the plurality of potentiometric sensor units has a shape thatdefines a lateral projection on the surface of the electrolyte.
 18. Themicrofabricated potentiometric gas sensor device of claim 1, wherein thelayer of the WO₃ of at least one of the plurality of potentiometricsensor units has a shape that defines a plurality of lateral projectionson the surface of the electrolyte.
 19. The microfabricatedpotentiometric gas sensor device of claim 18, wherein at least one ofthe plurality of lateral projections of WO₃ has at least two edgeinterfaces along the surface of the electrolyte layer.
 20. Themicrofabricated potentiometric gas sensor device of claim 1, wherein thesurface area of the platinum (Pt) contact is at least 5 times smallerthan the surface area of the layer of tungsten oxide (WO₃).
 21. Themicrofabricated potentiometric gas sensor device of claim 1, wherein thesurface area of the platinum (Pt) contact is at least 9 times smallerthan the surface area of the sensing electrode.
 22. The microfabricatedpotentiometric gas sensor device of claim 1 further comprising: a firstelectrical interconnect coupled to the sensing electrode of a firstpotentiometric sensor within the series of potentiometric sensor units;a second electrical interconnect electrically coupled to the referenceelectrode of a last potentiometric sensor within the series ofpotentiometric sensors; and wherein the microfabricated potentiometricgas sensor device is capable of measuring a combined potentialdifference at the first and second electrical leads.
 23. Themicrofabricated potentiometric gas sensor device of claim 22, furthercomprising a third potentiometric sensor unit electrically coupledbetween the first and last potentiometric sensor units within the seriesof potentiometric sensors, wherein the sensing electrode of the firstpotentiometric sensor is connected to the reference electrode of thethird potentiometric sensor.
 24. The microfabricated potentiometric gassensor device of claim 23, wherein the combined potential differencecomprises a sum of the potential differences of the first, the secondand the third potentiometric sensor units.
 25. The microfabricatedpotentiometric gas sensor device of claim 1, wherein the sensor deviceis made using the photolithography fabrication method.
 26. Themicrofabricated potentiometric gas sensor gas sensor device of claim 1,wherein the sensor device is made using the shadow mask fabricationmethod.
 27. The microfabricated potentiometric gas sensor device ofclaim 1, wherein the sensor device was fabricated using photolithographymethod, wherein the surface of the sensor unit has been annealed withoxygen to remove a photoresist mask.
 28. The microfabricatedpotentiometric gas sensor device of claim 1, wherein the sensor devicecomprises 15 to 20 sensor units.
 29. The microfabricated potentiometricgas sensor device of claim 1, wherein the wherein the surface area ofthe WO₃ electrode on the electrolyte is at least five times greater thanthe surface area of the Pt electrode on the electrolyte, and the layerof the WO₃ has a shape that defines a plurality of lateral projectionson the surface of the electrolyte.
 30. The microfabricatedpotentiometric gas sensor device of claim 29, wherein the surface areaof the platinum (Pt) contact is at least 5 times smaller than thesurface area of the layer of tungsten oxide (WO₃) and the electrolytecomprises yttria-stabilized zirconia (YSZ).
 31. A microfabricatedpoteniometric sensor device for detecting gas comprising: a basesubstrate; an electrolyte layer disposed on the base substrate; aplurality of potentiometric sensor units connected in series and coupledto the base substrate, each potentiometric sensor unit comprising: anelectrolyte layer disposed on the base substrate; a two-part sensingelectrode comprising a layer of tungsten oxide (WO₃) disposed on aplatinum (Pt) contact; a reference electrode comprising platinum (Pt);and wherein the ratio of the surface area of the WO₃ sensing electrodedisposed on the electrolyte to the surface area of the Pt referenceelectrode disposed on the electrolyte is sufficiently high such that themicrofabricated potentiometric gas sensor device is capable of detectingthe gas level at a sensitivity of at least 1 ppm.
 32. Themicrofabricated potentiometric gas sensor device of claim 31, whereinthe gas comprises NO_(x).
 33. The microfabricated potentiometric gassensor device of claim 32, wherein the sensor device determines the gaslevel of NO_(x) at a sensitivity of at least 300 ppb.
 34. Themicrofabricated potentiometric gas sensor device of claim 33, whereinthe layer of the WO₃ of the sensing electrode of at least one of theplurality of potentiometric sensor units has a shape defining aplurality of lateral projections on the surface of the electrolyte. 35.A microfabricated poteniometric sensor device for detecting gascomprising: a base substrate; an electrolyte layer disposed on the basesubstrate; a plurality of potentiometric sensor units connected inseries and coupled to the base substrate, each potentiometric sensorunit comprising: an electrolyte layer disposed on the base substrate; atwo-part sensing electrode comprising a layer of tungsten oxide (WO₃)disposed on a platinum (Pt) contact; a reference electrode comprisingplatinum (Pt); and wherein the layer of the WO₃ of the sensing electrodeof at least one of the plurality of potentiometric sensor units has ashape that defines a lateral projections on the surface of theelectrolyte.
 36. The microfabricated potentiometric gas sensor device ofclaim 35, wherein the electrolyte electrolyte comprisesyttria-stabilized zirconia (YSZ) and the sensor device is capable ofdetermining the gas level of NO_(x) at a sensitivity of at least 1 ppm.37. A microfabricated potentiometric gas sensor device comprising: abase substrate; an electrolyte layer disposed on the base substrate; aplurality of potentiometric sensor units connected in series and coupledto the base substrate, each potentiometric sensor unit comprising: anelectrolyte layer disposed on the base substrate; a two-part sensingelectrode comprising a layer of tungsten oxide (WO₃) disposed on aplatinum (Pt) contact; a reference electrode comprising platinum (Pt);and wherein a thickness of the electrolyte layer is maximized asufficient amount to minimize the internal resistance of thecorresponding potentiometric sensor unit, and such that the internalresistance of each of the plurality of potentiometric sensor units isminimized so as to minimize the overall resistance of themicrofabricated potentiometric gas sensor device and increasesensitivity of the microfabricated potentiometric gas sensor device to agas level sensitivity of at least 1 ppm.